III-Nitride compound semiconductor light emitting device

ABSTRACT

The present invention provides a III-nitride compound semiconductor light emitting device comprising an active layer ( 30 ) which emits light and is interposed between a lower contact layer ( 20 ) made of n-GaN and an upper contact layer ( 40 ) made of p-GaN, in which a sequential stack of a lattice mismatch-reducing layer L 3  made of In x Ga 1-x N, an electron supply layer L 4  made of n-GaN or n-Al y Ga 1-y N and a crystal restoration layer L 5  made of In z Ga 1-z N is interposed between the lower contact layer and the active layer, and further comprising an electron acceleration layer L 1  made of n-GaN or undoped GaN and a heterojunction electron barrier-removing layer L 2 , thereby the lattice mismatch between the lower contact layer ( 20 ) and the active layer ( 30 ) can be reduced.

TECHNICAL FIELD

The present invention relates to a III-nitride compound semiconductorlight emitting device, and more particularly, to a III-nitride compoundsemiconductor light emitting device in which the lattice mismatchbetween an active layer and a lower contact layer made of n-GaN can bereduced and a reduction in thin film quality caused by the doping of ann-type dopant can be prevented.

BACKGROUND ART

FIG. 1 is a cross-sectional view for illustrating a III-nitride compoundsemi-conductor light emitting device according to the prior art.

Referring to FIG. 1, the prior III-nitride compound semiconductor lightemitting device is fabricated in the following manner. The buffer layer15 made of undoped GaN, the lower contact layer 20 made of n-GaN, theactive layer 30 with a single-quantum-well structure or amultiple-quantum-well structure, and the upper contact layer 40 made ofp-GaN, are sequentially deposited on the sapphire substrate 20. Then,mesa etching is performed in such a way to expose the lower contactlayer 20. After the electrode layers 50, 60 and 70 to be used aselectrodes are formed, the passivation film 80 is formed.

The multiple-quantum-well structure of the active layer 30 is composedof an alternate stack of quantum well layers and quantum barrier layers,in which the quantum well layers may be made of InGaN, and the quantumbarrier layers may be InGaN or GaN. It is understood that, if all thequantum well layers and the quantum barrier layers are made of InGaN,the amount of indium (In) in the quantum barrier layers will be lowerthan the amount of indium in the quantum well layers.

The above-described prior III-nitride compound semiconductor lightemitting device has a problem in that the lattice mismatch between thelower contact layer 20 and the active layer 30 is very large. The longerthe wavelength of light to be emitted becomes, the larger the latticemismatch becomes, thus making it more difficult to grow the active layer30 with high quality. The III-nitride compound semiconductor has theproperty of piezoelectrics, which becomes stronger as the latticemismatch becomes larger. Particularly, when the piezoelectric phenomenonoccurs within the active layer 30, a change of the energy band shape ofthe active layer 30 will be caused, resulting in the distortion of thewave functions of holes and electrons, thus reducing light emittingefficiency in the active layer 30.

Meanwhile, as the concentration of n-type dopants (e.g., silicon) in theIII-nitride compound semiconductor becomes higher, the thin film qualityof the III nitride compound semiconductor layer is deteriorated, andbecause of its influence, a thin film layer deposited thereon will alsobe deteriorated in quality. As a result, as the doping concentration ofthe lower contact layer 20 becomes higher, the thin film quality of theactive layer adjacent thereto will be deteriorated, resulting inreductions in the thin film quality of the active layer and thereliability of a device.

As described above, in the prior III-nitride compound semiconductorlight emitting device, there are the problem of the lattice mismatchbetween the lower contact layer 20 and the active layer 30, and theproblem of a reduction in the qualities of the lower contact layer 20and the active layer 30, which is caused by n-type doping to the lowercontact layer 20.

DISCLOSURE OF INVENTION

Technical Problem

Accordingly, it is an object of the present invention to provide aIII-nitride compound semiconductor light emitting device in whichelectrons can be effectively supplied to the active layer 30, and at thesame time, the mismatch between the lower contact layer 20 and theactive layer 30 can be reduced, and a reduction in the quality of thelower contact layer 20 and a thin film formed thereon, which is causedby n-type doping to the lower contact layer 20, can be prevented.

Technical Solution

To achieve the above object, the present invention provides aIII-nitride compound semiconductor light emitting device comprising anactive layer which emits light and is interposed between a lower contactlayer made of n-GaN and an upper contact layer made of p-GaN, in which asequential stack of a lattice mismatch-reducing layer made ofIn_(x)Ga_(1-x)N, an electron supply layer made of n-GaN orn-Al_(y)Ga_(1-y)N and a crystal restoration layer made ofIn_(z)Ga_(1-z)N is interposed between the lower contact layer and theactive layer.

The active layer may have a single-quantum-well or multiple-quantum-wellstructure comprising quantum well layer made of In_(x)Ga_(1-x)N.

The active layer may have a multiple-quantum structure composed of analternate stacking of quantum well layers and quantum barrier layers, inwhich the lattice mismatch-reducing layer preferably has an energy bandgap larger than the energy band gap of the quantum well layers andsmaller than the energy band gap of the quantum barrier layers.

The lattice mismatch-reducing layer preferably has a thickness of10-1000 Å.

The lattice mismatch-reducing layer is preferably undoped.

The indium content of the lattice mismatch-reducing layer is preferably0<x≦0.4.

The Al content of the electron supply layer is preferably 0<y≦0.2.

The electron supply layer preferably has a thickness of 10-500 Å.

The doping concentration of the electron supply layer is preferably5×10¹⁷-10×10²¹ atoms/cm³.

The active layer may have a multiple-quantum-well structure composed ofan alternate stacking of quantum well layers and quantum barrier layers,in which the crystal restoration layer preferably has an energy band gaplarger than the energy band gap of the quantum well layers and smallerthan the energy bandgap of the quantum barrier layers.

The crystal restoration layer preferably has a thickness of 10-500 Å.

The crystal restoration layer is preferably undoped.

The indium content of the crystal restoration layer is preferably0<z≦0.4.

Between the lower contact layer and the lattice mismatch-reducing layer,a sequential stack of an electron acceleration layer made of n-GaN orundoped GaN and a heterojunction electron barrier-removing layer made ofa higher doping concentration of n-GaN than that of the electronacceleration layer may be interposed.

When the electron acceleration layer is made of n-GaN, the dopingconcentration of the electron acceleration layer is preferably1×10¹⁵-1×10¹⁸ atoms/cm³.

The electron acceleration layer preferably has a thickness of 100-10000Å.

The doping concentration of the heterojunction electron barrier-removinglayer is preferably 1×10¹⁸-1×10²¹ atoms/cm³.

The heterojunction electron barrier-removing layer preferably has athickness of 10-300 Å.

The heterojunction electron barrier-removing layer may be a delta-dopinglayer.

Meanwhile, the heterojunction electron barrier-removing layer may alsobe composed of an alternate stack in a superlattice form of a firstlayer made of n-GaN having a higher doping concentration than that ofthe electron acceleration layer and a second layer made of undoped GaNor n-GaN having a lower doping concentration than that of the firstlayer. In this case, the doping concentration of the first layer ispreferably 1×10¹⁸-1×10²¹ atoms/cm³. Furthermore, the thickness of eachof the first and second layer is preferably 5-150 Å and the thickness ofthe heterojunction electron barrier-removing layer is preferably 20-500Å.

Advantageous Effects

According to the present invention as described above, electrons can beeffectively supplied to the active layer 30, and at the same time, thelattice mismatch between the lower contact layer 20 and the active layer30 can be reduced and a reduction in the quality of the lower contactlayer 20 and a thin film formed thereon, which occurs due to n-typedoping to the lower contact layer 20, can be prevented.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view illustrating the prior III-nitridecompound semi-conductor light emitting device.

FIG. 2 is a cross-sectional view illustrating a III-nitride compoundsemiconductor light emitting device according to an embodiment of thepresent invention.

FIG. 3 is the energy-band diagram of the conduction band of each layerof the light emitting device shown in FIG. 2.

FIG. 4 is a graph showing the concentration of n-type dopant silicon ineach of the layers shown in FIG. 2.

FIG. 5 is a heterojunction energy-band diagram in the case the lowercontact layer 20 is in a direct contact with the latticemismatch-reducing layer L3.

FIG. 6 is an energy-band diagram illustrating the roles of the electronacceleration layer L1 and the heterojunction electron barrier-removinglayer L2.

FIG. 7 is a graph showing light output as a function of the thickness ofthe lattice mismatch-reducing layer L3.

FIG. 8 is a graph showing light output as a function of the indium ofthe lattice mismatch-reducing layer L3.

FIG. 9 is a graph showing light output as a function of the dopingconcentration of the electron supply layer L4.

FIG. 10 is a graph showing leakage current as a function of thethickness of the crystal restoration layer L5.

FIG. 11 is a graph showing forward driving voltage as a function of thedoping concentration of the heterojunction electron barrier-removinglayer L2.

MODE FOR THE INVENTION

Hereinafter, the present invention will be described in detail withreference to the accompanying drawings. In the drawings, the samereference numerals as those in FIG. 1 represent elements performing thesame functions, and a duplicate description thereof will be omitted.

The following embodiments are provided for a better understanding of thepresent invention and it will be obvious to any person skilled in theart that many modifications to these embodiments can be made withintechnical concept of the present invention. Accordingly, it should notbe construed that the scope of the present invention is not limited toor by these embodiments.

FIG. 2 is a cross-sectional view illustrating a III-nitride compoundsemiconductor light emitting device according to an embodiment of thepresent invention.

Referring to FIG. 2, between the lower contact layer 20 and the activelayer 30, the lattice mismatch-reducing layer L3 made of undopedIn_(x)Ga_(1-x)N to reduce strain caused by lattice mismatch isinterposed. On the lattice mismatch-reducing layer L3, the electronsupply layer L4 made of n-Al_(y)Ga_(1-y)N to effectively supplyelectrons to the active layer 30 is deposited. On the electron supplylayer L4, the crystal restoration layer L5 made of undopedIn_(z)Ga_(1-z)N to restore the quality of the electron supply layer L4whose quality has been deteriorated due to doping is deposited. Thus,the active layer 30 is adjacent to the crystal restoration layer L5.

If necessary, in order to improve the operating properties of theinventive device having the lattice mismatch-reducing layer L3, asequential stack of the electrode acceleration layer L1 made of n-GaN orundoped GaN and the heterojunction electron battier layer L2 made ofn-GaN may also be interposed between the lower contact layer 20 and thelattice mismatch-reducing layer-reducing layer L3.

FIG. 3 is the energy-band diagram of the conduction band of each layerof the device shown in FIG. 2, which was schematically plotted on thebasis of the composition of the layer without considering the dopingconcentration of the layer. And, FIG. 4 shows the concentration ofn-type dopant silicon in each of the layers shown in FIG. 2.

Referring to FIGS. 3 and 4, the lattice mismatch-reducing layer L3 andthe crystal restoration layer L5 has an energy band gap larger than theenergy band gap of the quantum well layers W and smaller than the energyband gap of the quantum barrier layer B. However, this does not meansthat the lattice mismatch-reducing layer L3 and the crystal restorationlayer L5 should have energy band gaps of the same size. The energy bandgaps of the lattice mismatch-reducing layer L3 and the crystalrestoration layer L5 vary depending on the amount of indium and becomesmaller as the amount of indium increases. The indium content of thecrystal restoration layer L5 is preferably 0<z≦0.4.

When the lattice mismatch-reducing layer L3 and the crystal restorationlayer L5 are doped, there will be a high possibility that their qualityis deteriorated, thus causing problems in the reliability and lightemitting efficiency of the device. For this reason, it is preferablethat the lattice mismatch-reducing layer L3 and the crystal restorationlayer L5 should not be intentionally doped.

In the present invention, the electron supply layer L4 whose quality hasbeen deteriorated due to n-type doping is not directly adjacent to theactive layer 30, but the crystal restoration layer L5 with no doping andthus with good quality is adjacent to the active layer 30. This allowsthe active layer 30 to be obtained with high quality, resulting in anincrease in the light emitting efficiency of the active layer 30. FIG.10 is a graph showing leakage current as a function of the thickness ofthe crystal restoration layer L5. As can be seen in FIG. 10, thethickness of the crystal restoration layer is preferably 10-500 Å andmore preferably 150-500 Å.

If the lattice mismatch-reducing layer L3 is excessively thick, therewill be a problem in that the series resistance of the device isincreased, leading to an increase in the driving voltage of the device,because the layer L3 has not been intentionally doped. On the otherhand, if the lattice mismatch-reducing layer L3 is excessively thin,there will be a problem in reducing the lattice mismatch between thelower contact layer 20 and the active layer 30. For these reason, thelattice mismatch-reducing layer L3 preferably has a thickness of 10-1000Å.

Test results for a change in the light output of the light emittingdevice with a change in the thickness of the lattice mismatch-reducinglayer L3 are shown in FIG. 7. These results were obtained at a lightemitting wavelength around 470 nm for the case where the latticemismatch-reducing layer L3 has an indium content of 3%. As can be seenin FIG. 7, the light emitting device shows the highest light output whenthe lattice mismatch-reducing layer L3 has a thickness of 500-700 Å atwhich the brightness is improved to a maximum of 50% as compared to thecase out of this optimum thickness range.

FIG. 8 shows light output as a function of the indium content of thelattice mismatch-reducing layer L3 when the thickness of the layer L3 is500 Å. As can be seen in FIG. 8, the light emitting device shows thehighest light emitting efficiency at an indium content of about 3-4%.

The thickness and indium content of the lattice mismatch-reducing layerL3 may have a variety of combinations depending on the degree of latticemismatch between the lower contact layer 20 and the active layer 30. Asthe bandgap of the active layer 30 becomes smaller, it is preferred thatthe thickness and indium content of the lattice mismatch-reducing layerL3 have larger values.

Although the introduction of the lattice mismatch-reducing layer L3 canprovide the effect of improving the thin film properties of the activelayer 30 by minimizing the lattice mismatch between the lower contactlayer 20 and the active layer 30, it will cause the following twoproblems.

First, since the lattice mismatch-reducing layer L3 has poor electricalconductivity because it has not been intentionally doped, electrons willfeel a difficulty in migration from the lower contact layer 20 to theactive layer 30. In other words, the number of electrons injected intothe active layer 30 will be decreased, leading to a reduction in lightemitting efficiency.

Second, since the lattice mismatch-reducing layer L3 contains indium,the contact surface between the lattice mismatch-reducing layer L3 andthe lower contact layer 20 below thereof will show heterojunctionproperties. Due to the heterojunction, an electron energy barrier willbe formed on the contact surface, so that the flow of electrons from thelower contact layer 20 to the active layer 30 will be obstructed. Thiswill lead to an increase in driving voltage.

To solve the first problem, the electron supply layer L4 made ofn-Al_(y)Ga_(1-y)N is formed on the lattice mismatch-reducing layer L3.

The concentration of the n-type dopant in the electron supply layer L4is preferably 5×10¹⁷-1×10²¹ atoms/cm³, and the thickness of the electronsupply layer L4 is preferably 10-500 Å. And the Al content of the layerL4 is preferably 0≦y≦0.2. The more the amount of Al becomes, the largerthe bandgap becomes, and the lattice mismatch-reducing layer L3 and thecrystal restoration layer L5, which are disposed above and below theelectron supply layer L4, respectively, are all made of InGaN. Thus, inorder to minimize the inconsistency between these two layers and theelectron supply layer L4 in electron conduction band, the band gap ofthe electron supply layer L4 is limited to equal or greater value thanthe bandgap of GaN as described above.

The introduction of the electron supply layer L4 is for solving theinterference of electron supply to the active layer 13, which resultsfrom the introduction of the lattice mismatch-reducing layer L3, therebyimprovements in light output and driving voltage are achieved.

FIG. 9 is a graph showing light output characteristics as a function ofthe doping concentration of the electron supply layer L4. As shown inFIG. 9, at low doping concentration, the light output shows a tendencyto decrease rapidly, which is a phenomenon resulting from insufficientelectron supply caused by the lattice mismatch-reducing layer L3. It canbe seen that the light output becomes optimal when the dopingconcentration of the electron supply layer L4 is about 5×10¹⁸. Atexcessively high doping concentration, the light output shows a tendencyto decrease, which seems to be a phenomenon occurring because adeterioration in thin film caused by excessive doping influences theactive layer 30.

Meanwhile, in order to solve the second problem, the heterojunctionelectron barrier-removing layer L2 made of highly doped n-GaN is formedbelow the lattice mismatch-reducing layer L3. The heterojunctionelectron barrier layer L2 is a delta doping layer having a higher dopingconcentration than that of the electron acceleration layer L1, whichserves to remove an electron barrier caused by the heterojunctionbetween the lower contact layer 20 and the lattice mismatch-reducinglayer.

The electron barrier by the heterojunction causes a problem in that itinterferes with the flow of electrons from the lower contact layer 20 tothe active layer 30, leading to a great increase in the driving voltageof the device. This problem becomes more serious as the indium contentof the lattice mismatch L3 increases.

The heterojunction electron barrier-removing layer L2 preferably has adoping concentration of 1×10¹⁸-1×10²¹ atoms/cm³, and a thickness of10-300 Å. The heterojunction electron barrier-removing layer L2 shouldbe very thin, and this thin thickness can be achieved by delta doping.This is because if the heterojunction electron barrier-removing layer L2is thick, its quality will be deteriorated to make the properties of thethin film formed thereon poor, thus-deteriorating the performance of thedevice.

If necessary, the heterojunction electron barrier-removing layer L2 mayalso consist of an alternate stack in a superlattice form of a firstlayer made of n-GaN having a higher doping concentration than that ofthe electron acceleration layer L1 and a second layer made of GaN orn-GaN having a lower doping concentration than that of the first layer.In this case, the doping concentration of the first layers is preferably1×10¹⁸-1×10²¹ atoms/cm³. And, the thickness of each of the first andsecond layer is preferably 5-150 Å and the thickness of theheterojunction electron barrier-removing layer is preferably 20-500 Å.The construction of this supperlattice form has an advantage in that itcan improve the sensitivity of driving voltage to the thickness of theelectron barrier-removing layer.

FIG. 11 is a graph showing forward driving voltage at 20 mA as afunction of the doping concentration of the heterojunction electronbarrier-removing layer L2. As can be seen in FIG. 11, at low dopingconcentration, the driving voltage increases up to 4V, and depending onthe doping concentration, the driving voltage decreases up to 3.1 V. Forreference, the forward driving voltage of the nitride compoundsemiconductor light emitting device, which is currently needed, isgenerally equal to or below 3.5 V at 20 mA.

In order to increase the role of the heterojunction electronbarrier-removing layer L2, the electron acceleration layer L1 made oflow concentration n-GaN or intentionally undoped GaN is preferablydisposed below the heterojunction electron barrier-removing layer L2.

FIG. 5 is an energy band diagram for the case where the lower contactlayer 20 and the lattice mismatch-reducing layer L3 are in directcontact with each other. From FIG. 5, it can be seen that electrons aredifficult to pass to the lattice mismatch-reducing layer L3, due to thepresence of the electron barrier resulting from the heterojunction.

FIG. 6 is an energy band diagram illustrating the roles of the electronacceleration layer L1 and the heterojunction electron barrier-reducinglayer L2. As can be seen in FIG. 6, the energy band will be bentdownward due to a difference in doping concentration between theelectron acceleration layer L1 and the heterojunction electronbarrier-removing layer L2. Thus; the electric field will occur in theopposite direction to the arrow, and by this electric field, electronswill be accelerated toward the heterojunction electron barrier-removinglayer L2. Moreover, by the heterojunction electron barrier-removinglayer L2 having a high doping concentration, the height and width of theelectron barrier caused by heterojunction will be reduced. This, by suchtwo effects, electrons will be easily passed through the heterojunctionelectron barrier, resulting in a significant reduction in the drivingvoltage of the device.

For nitride compound semiconductors, at high n-type dopingconcentration, the thin film quality shows a tendency to decreaserapidly. For this reason, it is preferable that the electronacceleration layer L1 should not be intentionally doped or should bedoped at a low concentration. when the electron acceleration layer L1 isdoped, it will preferably have a doping concentration of 1×10¹⁵-1×10¹⁸atoms/cm³ and a thickness of 10-10000 Å.

1. A III-nitride compound semiconductor light emitting devicecomprising: an active layer emitting light and being interposed betweena lower contact layer made of n-GaN and an upper contact layer made ofp-type III-nitride compound semiconductor layer, the active layer havingat least one quantum well layer and one quantum barrier layer in contactwith the quantum well layer, an n-type electrode layer formed on thelower contact layer, a lattice mismatch-reducing layer made ofIn_(x)Ga_(1-x)N(x>0), grown on the lower contact layer and having athickness of 200-1000 Å, the lattice mismatch-reducing layer having anenergy band gap larger than the energy band gap of the quantum welllayer and smaller than the energy band gap of the quantum barrier layer,an electron supply layer made of n-Al_(y)Ga_(1-y)N(y≧0) and grown on thelattice mismatch-reducing layer, and a crystal restoration layer made ofIn_(z)Ga_(1-z)N(z>0), grown on the electron supply layer and in contactwith the active layer.
 2. The III-nitride compound semiconductor lightemitting device of claim 1, wherein the active layer has asingle-quantum-well or multiple-quantum-well structure comprisingquantum well layer made of In_(x)Ga_(1-x)N.
 3. The III-nitride compoundsemiconductor light emitting device of claim 1, wherein the latticemismatch-reducing layer is undoped.
 4. The III-nitride compoundsemiconductor light emitting device of claim 1, wherein the indiumcontent of the lattice mismatch-reducing layer is 0<x≦0.4.
 5. TheIII-nitride compound semiconductor light emitting device of claim 1,wherein the Al content of the electron supply layer is 0<y≦0.2.
 6. TheIII-nitride compound semiconductor light emitting device of claim 1,wherein the electron supply layer has a thickness of 10-500 Å.
 7. TheIII-nitride compound semiconductor light emitting device of claim 1,wherein the doping concentration of the electron supply layer is5×10¹⁷-10×10²¹ atoms/cm³.
 8. The III-nitride compound semiconductorlight emitting device of claim 1, wherein the crystal restoration layerhas an energy band gap larger than the energy band gap of the quantumwell layer and smaller than the energy bandgap of the quantum barrierlayer.
 9. The III-nitride compound semiconductor light emitting deviceof claim 1, wherein the crystal restoration layer has a thickness of10-500 Å.
 10. The III-nitride compound semiconductor light emittingdevice of claim 1, wherein the crystal restoration layer is undoped. 11.The III-nitride compound semiconductor light emitting device of claim 1,wherein the indium content of the crystal restoration layer is 0<z≦0.4.12. A III-nitride compound semiconductor light emitting devicecomprising: an active layer emitting light and being interposed betweena lower contact layer made of n-GaN and an upper contact layer made ofp-type III-nitride compound semiconductor layer, an n-type electrodelayer formed on the lower contact layer, a lattice mismatch-reducinglayer made of In_(x)Ga_(1-x)N(x>0), grown on the lower contact layer andhaving a thickness of 200-1000 Å, an electron supply layer made ofn-Al_(y)Ga_(1-y)N(y≧0) and grown on the lattice mismatch-reducing layer,a crystal restoration layer made of In_(z)Ga_(1-z)N(z>0), grown on theelectron supply layer and in contact with the active layer, an electronacceleration layer made of n-GaN or undoped GaN and grown on the lowercontact layer, and a heterojunction electron barrier-removing layer madeof a higher doping concentration of n-GaN than that of the electronacceleration layer and grown on the electron acceleration layer, whereinthe lattice mismatch-reducing layer is grown on the heterojunctionelectron barrier-removing layer.
 13. The III-nitride compoundsemiconductor light emitting device of claim 12, wherein the dopingconcentration of the electron acceleration layer is 1×10¹⁵-1×10¹⁸atoms/cm³ when the electron acceleration layer is made of n-GaN.
 14. TheIII-nitride compound semiconductor light emitting device of claim 12,wherein the electron acceleration layer has a thickness of 100-10000 Å.15. The III-nitride compound semiconductor light emitting device ofclaim 12, wherein the doping concentration of the heterojunctionelectron barrier-removing layer is 1×10¹⁸-1×10²¹ atoms/cm³.
 16. TheIII-nitride compound semiconductor light emitting device of claim 12,wherein the heterojunction electron barrier-removing layer has athickness of 10-300 Å.
 17. The III-nitride compound semiconductor lightemitting device of claim 12, wherein the heterojunction electronbarrier-removing layer is a delta-doping layer.
 18. The III-nitridecompound semiconductor light emitting device of claim 1, wherein asequential stack of an electron acceleration layer made of n-GaN orundoped GaN and a heterojunction electron barrier-removing layer isinterposed between the lower contact layer and the latticemismatch-reducing layer, and the heterojunction electronbarrier-removing layer is composed of an alternate stack in asuperlattice form of a first layer made of n-GaN having a higher dopingconcentration than that of the electron acceleration layer and a secondlayer made of undoped GaN or n-GaN having a lower doping concentrationthan that of the first layer.
 19. The III-nitride compound semiconductorlight emitting device of claim 18, wherein the thickness of each of thefirst and second layer is 5-150 Å and the thickness of theheterojunction electron barrier-removing layer is 20-500 Å.
 20. AIII-nitride compound semiconductor light emitting device comprising: afirst layer made of n-GaN and having a first doping concentration, anelectrode in electrical contact with the first layer for supplyingelectrons to the first layer, a p-type III-nitride compoundsemiconductor layer, an active layer emitting light, being interposedbetween the first layer and the p-type III-nitride compoundsemiconductor layer and having at least one quantum well layer and onequantum barrier layer in contact with the quantum well layer, a latticemismatch-reduction layer made of In_(x)Ga_(1-x)N(x>0) interposed betweenthe first layer and the active layer and having an energy band gaplarger than an energy band gap of the quantum well layer and smallerthan an energy band gap of the barrier layer, and a second layer made ofn-GaN having a second doping concentration larger than the first dopingconcentration for removing the heterojunction electron barrier betweenthe first layer made of n-GaN and the lattice mismatch-reduction layermade of In_(x)Ga_(1-x)N(x>0).
 21. The III-nitride compound semiconductorlight emitting device of claim 20, comprising: a third layer made ofn-GaN or undoped GaN interposed between and in contact with the firstlayer and the second layer and having a third doping concentrationsmaller the first doping concentration.
 22. The III-nitride compoundsemiconductor light emitting device of claim 21, wherein the latticemismatch-reduction layer is undoped.
 23. The III-nitride compoundsemiconductor light emitting device of claim 20, wherein the seconddoping concentration is 1×10¹⁸-1×10²¹ atoms/cm³.
 24. The III-nitridecompound semiconductor light emitting device of claim 20, wherein thesecond layer has a thickness of 10-300 Å.
 25. The III-nitride compoundsemiconductor light emitting device of claim 20, wherein the secondlayer is a delta-doping layer.